Conventional gas turbine engines include compressors for pressurizing air which is then mixed with fuel and ignited to undergo combustion with the resulting combustion gases being channeled through a turbine. The compressor and turbine are surrounded by casings through which air is either extracted or distributed.
More specifically, in a compressor, a portion of the air compressed therein is typically extracted as bleed air for conventional use in either an aircraft being powered by the engine, or in the turbine for obtaining preferential cooling thereof. The bleed air extracted from the compressor or distributed to the turbine is typically channeled through a plurality of circumferentially spaced apart air ports in the respective casings.
In the compressor, an inner casing or flowpath is typically spaced radially inwardly from the outer casing and surrounds conventional rotor blades and stator vanes which are mounted thereto. The inner casing includes a plurality of circumferentially spaced conventional bleed holes which channel a portion of the air compressed by the blades into the plenum defined between the outer and inner casings. The bleed air then flows circumferentially in the plenum to the nearest air port through which it is discharged from the compressor. The velocity of the bleed air is relatively low between adjacent ones of the air ports and relatively high at each of the air ports through which the air is being funneled out of the compressor.
Accordingly, the heat transfer capability of the bleed air, which is directly proportional to the velocity thereof, is relatively low between the air ports and relatively high at the air port. This variation in heat transfer capability of the bleed air leads to circumferential thermal distortion of the outer casing. For example, as the bleed air heats the outer casing, the outer casing at the air ports will expand more than the casing between the air ports causing the outer casing to become out of round. Since the inner casing is supported by the outer casing, the inner casing also becomes out of round which is undesirable since the inner casing surrounds the rotor blades, and increased clearance between the blade tips and the inner casing results in a decrease of compressor efficiency.
In the turbine, the bleed air is channeled therein through a plurality of circumferentially spaced apart air ports for circumferential distribution therein. Similarly, the velocity of the bleed air channeled into the turbine is greatest as it flows through the air ports and decreases between the air ports. Similar circumferential thermal distortion also results in the turbine casing.